When driving on the interstate that's exactly when I want a device like this. The screen is situated outside your normal viewing area, and for this application is normally tranparent. There's a device already for sale that mounts on your motorcycle helmet to gives you speed, rpm, and gear information in a HUD. It's a LOT safer to look at a screen in the upper right corner of your vision that's set at the same focal distance as the road than to have to look down at a speed display, refocus, read the display, look back at the road, and refocus again.
I think Google is painting a cool vision of a future device... but it's a fair distance into the future (5 years or more I'd estimate) before device size gets small enough, resolution gets high enough, battery life gets long enough, and price gets low enough to provide a viable product in this market.